The present invention relates to an electronic balance and more particularly to an electronic balance of the electromagnetic force balancing type.
In an electronic balance of the electromagnetic force balancing type, an electromagnetic force generated at the time when a current flows in a force coil disposed in a magnetic field, is used as a counterbalance force with respect to the weight of a load to be measured, and the weight of such a load is obtained based on the value of a current required for obtaining an equilibrium between the load weight and the electromagnetic force. The electronic balance of the type above-mentioned may be divided into the following types according to the method of supplying the current to the force coil, the method of measuring the current value and the like:
(1) By changing the duty of a pulse current of about 500 to 1000 Hz using one feedback loop, the electromagnetic force is balanced with the weight of a load, and the pulse width is measured by counting clock pulses passing therethrough.
(2) A pulse current for which any of predetermined N-step duties can be selected, flows in the force coil so that the current is roughly balanced with the weight of a load, and the remaining weight deviation is balanced by a servo system. The value of the current flowing in the force coil is measured by weighted-addition of the pulse duty value selected and the value as obtained by A-D conversion of a PID output in the servo system to each other.
(3) The entire weight of a load is balanced with an electromagnetic force generated by a DC servo system, and the value of a current flowing at the time when such a balance is obtained, is A/D converted.
Out of the conventional methods above-mentioned, the method (1) is disadvantageous in view of limited resolution and response characteristics. More specifically, the cycle of a pulse current flowing in the force coil is limited to about max. 2 milliseconds due to the number of vibrations inherent in the balance mechanism. If the cycle exceeds 2 milliseconds, the balance beam is considerably vibrated. Accordingly, it is required to measure the pulse width changing in this cycle of 2 milliseconds by counting clock pulses. However, even though clock pulses of 30 MHz are used and counted, there is merely obtained max. 60,000 counts (about 16 bits on the binary scale) which is the limit of resolution to be obtained where general-purpose ICs are used.
According to the method (2) above-mentioned, the resolution can be improved. However, the method (2) presents the following problems. That is, in measurement of the weight of a load which undergoes a change from time to time, or in measurement of weighing-out or the like, the pulse duty determined in N steps is changed by one step at the moment when the weight exceeds the range to be measured by the servo system. At this time, there is temporarily produced an excessive force compensation due to the response characteristics of a PID control output of the servo system. This causes the balance mechanism to be swung so that the measured and displayed value is temporarily considerably changed.
The method (3) above-mentioned requires an A/D conversion function with high precision. This requires high stability for both the servo system and the A/D converter. Thus, both the resolution and the stability are hardly assured. In the analog servo mechanism, a certain limit is placed upon integration of the circuit thereof and a number of portions in the circuit should be adjusted. It is therefore difficult to uniformalize the respective performances of electronic balances each using such an analog servo mechanism.
In view of the foregoing, the inventor has already proposed an electronic balance in which (i) pulse currents flow in force coils, (ii) there is fetched, as a digital signal, a signal which represents a displacement of the load receiving member and which is detected for detecting the balancing state of the balance, (iii) this digital signal is subjected to digital PID operations (proportion, integration and differentiation processings), and (iv) the duties of pulse currents to flow in the force coils are determined based on the operation results (Japanese Patent Laid-Open Publication 3-63526).
In the electronic balance according to the proposal above-mentioned, there are generated a plurality of force coil currents respectively having different current values to which different pulse duty data are respectively given. Accordingly, a plurality of pulse currents respectively having different current values (peak values) and different duties are respectively supplied to the force coils. The plurality of pulse currents are made uniform during a predetermined period of control time, and supplied to the force coils in an overlapping state so as to bring the electromagnetic force into equilibrium with the load weight. Then, by weighted-adding these pulse currents flowing through the force coil for the predetermined period of time, the resolution is enhanced.
According to the method above-mentioned, even though the resolution (the number of bits) of each pulse current generating means is limited to a certain level, the total sum of respective resolutions is regarded as the resolution of the balance in its entirety. Thus, the resolution of the balance in its entirety can be advantageously improved as desired by increasing the number of pulse current generating means and the number of divisions of pulse duty data.
However, when this method is adopted in an electronic balance which does not require precision so much, the balance according to the method above-mentioned is disadvantageous in view of cost.